Electron transport studies in rhombohedral series of Al-doped LaMnO3+delta: an effective medium approach

We report the electron transport studies in the rhombohedral LaMn1-xAlx O3delta series (0 less than or equal to x less than or equal to 20%) of sampl es. Though all the samples are ferromagnetic below a transition temperature (T-C), only the samples with x less than or equal to 5% exhibit a metal-in sulator transition (MIT) at T-C while the samples with x greater than or eq ual to 10% are all semiconducting above and below T-C. The sample with x = 7.5% is a borderline case where MIT at T-C is immediately taken over by sem iconducting behaviour at lower temperatures. Thus a progressive crossover f rom ferromagnetic-metallic state to ferromagnetic-insulating state is obser ved in this series within the same structure. This is accompanied by a decr ease in Mn4+ content across the magnetic and conduction percolation thresho lds. This systematic crossover from a double exchange dominated regime to a n exclusively superexchange regime, preserving the rhombohedral symmetry, m akes the present series an important and unique one for the study of electr on transport in the colossal magnetoresistance manganites. An effective med ium approach is employed to explain the resistivity behaviour in this serie s over the whole temperature range, which gives strong support for polaroni c conduction in all the samples. This polaronic conduction justifies the pr esence of dynamic Jahn-Teller effects in the samples and the change in the character of polarons across T-C illustrates the role of electron-lattice i nteraction as well as its coupling to magnetic states (core spins) of the s amples. The dominance of double exchange is evident from metallic resistivi ty in the samples with x less than or equal to 5% which have Mn4+ much abov e the percolation threshold for metallicity. However the observed non-trivi al temperature dependence of metallic resistivity could not be explained by double exchange alone and it is shown in this study that one needs to take into account superexchange interactions even in the double exchange domina ted regime to understand the electron transport, thus supporting the curren t understanding in these pervoskite manganites.